The present invention relates to an objective lens used in an optical pickup apparatus wherein a light flux emitted from a laser is converged by an objective lens on an information recording plane through a transparent substrate of an optical information recording medium, and thereby an optical information is recorded and/or reproduced, and to the optical pickup apparatus. In particular, the invention relates to an objective lens and an optical pickup apparatus wherein first optical information recording medium having a t1-thick transparent substrate and second optical information recording medium having a t2-thick transparent substrate are used as an optical information recording medium, and a first light source having a wavelength of .lambda.1 is used for recording/reproducing the first optical information recording medium and a second light source having a wavelength of .lambda.2 is used for recording/reproducing the second optical information recording medium.
In recent years, with the practical use of a red semiconductor laser of a short wavelength, development of a DVD (digital video disk or called also a digital versatile disk), which is a high-density optical information recording medium having a larger capacity but a size of the same degree as a CD (compact disk) which is a conventional optical information recording medium (also called an optical disk), has been promoted. In this DVD, the numerical aperture NA of the objective lens at the side of the optical disk is made to be 0.6 in the case where a short wavelength semiconductor laser of 635 nm is used. Further, a DVD has a track pitch of 0.74 .mu.m and a minimum pit length of 0.4 .mu.m, and is made to have a high density more than twice in comparison with a CD, which has a track pitch of 1.6 .mu.m and a minimum pit length of 0.83 .mu.m. Moreover, in addition to the above-mentioned CD and DVD, optical disks having various standards, for instance, such as a CD-R (a direct read after write, writing once compact disk), a CD-RW, a LD (laser disk), an MD (mini-disk), and an MO (magneto-optic disk) are merchandised and have come into general use. In Table 5, the thickness of the transparent substrate and the required numerical aperture of various optical disks are shown.
TABLE 5 Thickness of Required transparent numerical aperture substrate NA (wavelength of Optical disk (mm) light source .lambda. nm) CD, CD-R 1.20 0.45 (.lambda. = 780) (reproducing only) CD-R 1.20 0.50 (.lambda. = 780) (recording, reproducing) LD 1.20 0.50 (.lambda. = 780) MD 1.20 0.45 (.lambda. = 780) MO (ISO 3.5 inch 230 MB) 1.20 0.55 (.lambda. = 780) MO (ISO 3.5 inch 640 MB) 1.20 0.55 (.lambda. = 680) DVD 0.60 0.60 (.lambda. = 635)
Besides, with regard to the CD-R, it is necessary for the light source to have a wavelength .lambda.=780 (nm), but for the other optical disks, a light source having a wavelength other than those noted in Table 5 can be used; in this case, the required numerical aperture NA should be varied in accordance with the wavelength .lambda. of the light source used. For example, in the case of a CD, the required numerical aperture is approximated by NA=.lambda. (.mu.m)/1.73, and in the case of a DVD, it is approximated by NA=.lambda. (.mu.m)/1.06.
Further, the numerical aperture as referred to in this specification (for example, referred to as NA1, NA2, NAL, NAH, NA3, NA4, etc. hereinafter) means the numerical aperture of the converging optical system as seen from the transparent substrate side.
As described in the above, it is now an age when various kinds of optical disks, which are different in the size, the thickness of the transparent substrate, the recording density, the wavelength used, and so forth, are available on the market, and optical pickup devices capable of being used for various kinds of optical disks are proposed.
As one of them, it has been proposed an optical pickup device which is equipped with converging optical systems corresponding to the different optical disks respectively, and the converging optical systems are switched over in accordance with the optical disk to be reproduced. However, in this optical pickup device, a plurality of converging optical systems is required and it brings not only a high cost but also the necessity of a driving mechanism for switching over the converging optical systems; accordingly, the device is not desirable because of the complexity and the requirement of the precision in switching over.
Therefore, various kinds of optical pickup devices which can reproduce a plurality of optical disks by using a single converging optical system are proposed.
The inventors have developed a special objective lens composed of a plurality of annular bands which are formed by dividing the whole surface into concentric circles, wherein each of the annular bands is corrected for the aberration within the diffraction limit with regard to each pertinent optical information recording medium by positively utilizing the spherical aberration produced owing to a plurality of light sources having different wavelength and/or the transparent substrates with different thickness of recording surface, and have previously proposed an optical pickup with a simplified structure using this objective lens (Japanese patent application H9-286954).
With the extension of CD-R (write-once compact disk) representing a programmable optical disk, an optical pickup apparatus is required to have interchangeability with the CD-R. Reflectance of the CD-R is lowered on the short wavelength side, and when the short wavelength red semiconductor laser is made to be a light source, necessary signals (reproduction signals, focus error signals, and tracking error signals) are not obtained. Therefore, a semiconductor laser having a wavelength of 780 nm is prepared for the CD-R, separately from the short wavelength semiconductor laser for DVD.
Further, in the recent years, there has been suggested a light source unit wherein these two semiconductor lasers mentioned above are unitized, for simplifying assembly of light source units and for improving work efficiency.
However, as will be explained in detail in an embodiment later referring to drawings, two semiconductor lasers are arranged side by side on mostly the same plane in a light source unit. Therefore, in the optical pickup apparatus employing the aforesaid light source unit, when the short wavelength semiconductor laser for DVD is set on the optical axis of a light converging optical system for recording/reproducing which is common to both light sources, the semiconductor laser for CD is out of the optical axis in the structure, and a light flux enters an objective lens obliquely.
Therefore, in the case of design wherein no sine condition dissatisfaction amount is generated for recording/reproducing of DVD as shown in aberration diagrams in FIG. 8, there are caused the following discrepancies, which was noticed by the inventor of the invention. Namely, in the case of using CD, an image height is usually owned, and thereby coma is in the serious state as shown in FIG. 9. In the case of using this unitized light source, when the sine condition is optimized in DVD, deterioration of coma is caused by image height and deterioration of astigmatism is caused by oblique incidence when CD is used, which results in unstable recording/reproducing operations of CD.
In this case, the sine condition dissatisfaction amount of the optical system excluding spherical aberration is defined as follows; EQU SC=DK.multidot.cos(uk)/NAk-(1-m).multidot.f EQU Nak=sin(uk')
wherein, PA1 m: lateral magnification of the optical system, PA1 f: focal length of the optical system, PA1 uk: angle made between an optical axis and a ray when the ray enters an optical system, PA1 uk': angle made between an optical axis and a ray when the ray emerges out of an optical system, and PA1 dk: height of a ray at the anterior principal point of an optical system. PA1 a first light source for emitting first light flux whose wavelength is 1; PA1 a second light source for emitting second light flux whose wavelength is .lambda.2, wherein .lambda.2 is greater than .lambda.1; PA1 a converging optical system comprising an objective lens having an optical axis, a first portion, a second portion, and a third portion, the third portion being farther from the optical axis than the first portion, the second portion being provided between the first portion and the second portion, and PA1 an Photo detector, PA1 wherein the first portion and the third portion are capable of converging the first light flux on a first information recording plane of a first optical information recording medium having a first transparent substrate of thickness t1 to reproduce information recorded in the first optical information recording medium or record information in the first optical information recording medium, PA1 wherein the first portion and the second portion are capable of converging the second light flux on a second information recording plane of a second optical information recording medium having a second transparent substrate of thickness t2 to reproduce information recorded in the second optical information recording medium or record information in the second optical information recording medium, wherein t2 is greater than t1, PA1 wherein the photo detector is capable of receiving tight flux reflected from the first information recording plane or the second information recording plane, and wherein the following conditional formula is satisfied for an amount of the first light flux not less than 30% of the total amount of the first light flux passing the first portion: EQU 0.06.gtoreq.SC1/f1.gtoreq.0.002 PA1 SC1: sine condition dissatisfaction amount of the objective lens at thickness t1 of a transparent substrate PA1 f1: focal length of the objective lens at thickness t1 of a transparent substrate. PA1 SC1: sine condition dissatisfaction amount of the objective lens at thickness t1 of a transparent substrate PA1 f1: focal length of the objective lens at thickness t1 of a transparent substrate. PA1 SC1: sine condition dissatisfaction amount of the objective lens at thickness t1 of a transparent substrate PA1 f1: focal length of the objective lens at thickness t1 of a transparent substrate. PA1 SC2: sine condition dissatisfaction amount of the objective lens at thickness t2 of a transparent substrate PA1 f2: focal length of the objective lens at thickness t2 of a transparent substrate. PA1 SC1: sine condition dissatisfaction amount of the objective lens at thickness t1 of a transparent substrate PA1 f1: focal length of the objective lens at thickness t1 of a transparent substrate. PA1 SC2: sine condition dissatisfaction amount of the objective lens at thickness t2 of a transparent substrate PA1 f2: focal length of the objective lens at thickness t2 of a transparent substrate. PA1 m1: Lateral magnification of a first portion of an objective lens in case that the first portion of the objective lens converges a first light flux on a first information recording plane PA1 f1: Focal length of a first portion of an objective lens in case that the first portion of the objective lens converges a first light flux on a first information recording plane PA1 uk1: Angle formed between an optical axis of an objective lens and a light flux at the aforesaid height among the first light flux entering the objective lens (for example, this angle is zero when the first light source is placed on an optical axis of the objective lens, and collimated light enters the objective lens) PA1 uk'1: Angle formed between an optical axis of an objective lens and a light flux at the aforesaid height among the first light flux emerging from the objective lens PA1 dk1: Height of a light flux at the anterior principal point of an objective lens
Therefore, when using a unitized light source, the sine condition for recording/reproducing DVD used on an axis is not optimized, but it is necessary that sine condition dissatisfaction amount SC which is on the level not to affect the axial capacity for recording/reproducing DVD adversely is given in advance in the course of the optical design, and thereby off-axis capacity for recording/reproducing CD is corrected.
Further, separately from the foregoing, there is suggested an optical pickup apparatus for recording/reproducing optical information recording media each having a transparent substrate with a different thickness wherein lateral magnification of an objective lens m is made to be greater than 0 for correcting spherical aberration in recording/reproducing of CD. When an objective lens is shifted from an optical axis for the reason of tracking, there is caused coma undesirably even in this case as an influence that the sine condition is not satisfied in the case of using CD. FIG. 10 shows an aberration diagram in the case of optimized sine condition for the use of DVD, and FIG. 11 shows image height characteristics of an objective lens. FIG. 11 shows the structure wherein image height characteristics of coma in the case of using CD are easily deteriorated.